topic 6 : the chemical senses

Question 1

What are the 5 taste qualities?

Answer 1

saltiness, sourness, sweetness, bitterness and umami

Question 2

how do we perceive different flavours?

Answer 2

-each food activates different combinations of basic taste, make it unique
-other sensory modalities contribute to a unique food-tasting experience –> e.g., texture, temperature, pain sensations for spice

Question 3

describe the different locations in which different tastes are most sensitive

Answer 3

Tip of the tongue is most sensitive to sweetness, the back to bitterness, and the sides to saltiness and sourness.

Question 4

What are papillae?

Answer 4

-scatter about the surface of the tongue, small projections
-shape like ridges (foliate papillae), pimples (vallate papillae) or mushrooms (fungiform papillae)
-each papillae has from one to several hundred taste bud

Question 5

what do taste buds have?

Answer 5

-Each taste bud has 50-150 taste receptor cells
-comprise only about 1% of tongue epithelium
-also has basal cells that surround the tastes, plus a set of gustatory afferent axons
-typical person has 2000-5000 taste buds

Question 6

What is the chemically sensitive part of the taste receptors?

Answer 6

small membrane region, the apical end, near surface of tongue

Question 7

describe the apical ends of gustatory

Answer 7

-have thin extensions called microvilli that project into taste pore (a small opening on the surface of the tongue where taste cell is exposed to the contents of the mouth)

Question 8

describe taste bud cell life span

Answer 8

Cells of the taste bud undergo a constant cycle of growth, death, and regeneration; the lifespan of one taste cell is about 2 weeks. This process depends on an influence of the sensory nerve, because if the nerve is cut, the taste buds will degenerate.

Question 9

What transmitters are used for tastes?

Answer 9

sour and salt –> serotonin
sweet, bitter and umami –> ATP

Question 10

what happens after receptor potential is large enough to depolarise taste receptor cells?

Answer 10

-Taste receptor’s transmitter excite postsynaptic gustatory axon –> communicate to brain stem.
-Taste cells may also use other transmitters, including acetylcholine, GABA, and glutamate, but their functions are unknown.

Question 11

What is transduction?

Answer 11

The process by which an environmental stimulus causes an electrical response in a sensory receptor cell

Question 12

what mechanisms are used by taste for transduction?

Answer 12

Taste stimuli, or tastants , may:
(1) directly pass through ion channels (salt and sour)
(2) bind to and block ion channels (sour)
(3) bind to G-protein coupled receptors in the membrane that activate second messenger systems that, in turn, open ion channels (bitter, sweet, and umami).

Question 13

describe the properties of saltiness

Answer 13

-prototypical salty chemical is table salt (NaCl)
-low concentrations (10-150 mM) taste good, but high concentrations are repellent
-taste of salt is taste of cation Na+

Question 14

How does the taste receptors detect low concentration of saltiness (Na+)

Answer 14

-To detect low concentrations, salt-sensitive taste cells use a special Na+-selective channel that is common in other epithelial cells and which is blocked by the drug amiloride. The amiloride-sensitive sodium channel is quite different from the voltage gated sodium channel that generates action potentials; the taste channel is insensitive to voltage and generally stays open. When you sip chicken soup, the Na + concentration rises outside the receptor cell, and the gradient for Na + across the membrane is made steeper. Na + then diffuses down its concentration gradient, which means it flows into the cell, and the resulting inward current causes the membrane to depolarize. This depolarization— the receptor potential— in turn causes voltage-gated sodium and calcium channels to open near the synaptic vesicles, triggering the release of neurotransmitter molecules onto the gustatory afferent axon.

Question 15

How does the taste receptors detect high concentration of saltiness (Na+)

Answer 15

-It appears that high salt levels activate bitter and sour taste cells, which normally trigger avoidance behaviours (not understood how).

Question 16

describe the properties of sourness

Answer 16

-taste of sourness comes from high acidity (low pH)
-can effect from inside or outside the taste cell membrane
-H+ bind to and block K+ channels –> permeability decreases
-H+ may also activate or permeate transient receptor potential (TRP) channels, thereby depolarise sour receptors

Question 17

How do the structures of bitter, sweet and umami tastes relate?

Answer 17

-all rely on T1R and T2R taste receptor proteins (G-protein-couple receptors)
-this is evidence that protein receptors for bitter, sweet and umami tastes are dimers (2 proteins affixed to one another)
-all use same second messenger pathway to carry their signal to afferent axons
-can be differentiated from each other because all three taste cell proteins expressed in different taste cell, and connect to different gustatory axons

Question 18

describe the unique properties of bitterness

Answer 18

-bitter substances detected by 25 different types of T2R receptors in humans –> poison detectors

Question 19

What is the general pathway of second messenger pathway when tastant binds to bitter, sweet or umami receptor?

Answer 19

-when binds activates G-proteins –>stimulates enzyme phospholipase C, thereby increasing production of intracellular messenger inositol triphosphate (IP3). –> activates special type of ion channel that is unique to taste cells, causing it to open and allow Na+ to enter and depolarise taste cell
-IP3 also triggers release of Ca2+ from intracellular storage sites. This Ca2+ in turn triggers transmitter release in an unusual way –>activate special membrane channel that allow ATP to flow out of cell. ATP serves as synaptic transmitter and activates purinergic receptors on postsynaptic gustatory axons

Question 20

Describe unique properties of sweetness

Answer 20

-require particular family of T1R receptors, T1R2 and T1R3 (both required to detect sweetness)

Question 21

what is the unique properties of umami (amino acids)?

Answer 21

-same T1R protein family as sweetness, T1R3
Has T1R1 and T1R3
-sweet and umami receptors share T1R3 protein so its T1R1 that determines whether receptor is sensitive to amino acids or sweet tastants
-when T1R1 removed unable to taste glutamate and other amino acids but can still demonstrate sense to sweet chemicals and other tastants

Question 22

What is the main flow of taste information?

Answer 22

from taste bud to primary gustatory axons, into brainstem, up to the thalamus and to the cerebral cortex

Question 23

how many nerves carry taste information and what are they?

Answer 23

-3 cranial nerves
-anterior two-thirds of the tongue and palate send axons into branch of cranial nerve VII (7, facial nerve)
-posterior third of the tongue is innervated by a branch of cranial nerve IX (9, glossopharyngeal nerve)
-regions around throat, including glottis, epiglottis and pharynx, send axons to the branch of cranial nerve X (10, vagus nerve)

Question 24

What do the nerves for taste synapse onto in the brainstem?

Answer 24

synapse within the slender gustatory nucleus, part of solitary nucleus in the medulla

Question 25

describe the path of the taste pathways in the neocortex after the pathways diverge from the gustatory nucleus

Answer 25

-gustatory nucleus synapse on subset of small neurons in ventral posterior medial (VPM) nucleus, portion of thalamus that deals with sensory information from the head
-VPM sends axons to primary gustatory cortex
-taste pathways to thalamus and cortex primarily ipsilateral to the cranial nerves that supply them

Question 26

what is the labelled line hypothesis?

Answer 26

-at the start of gustatory system (taste receptor cells) something like labelled lines are used. individual taste receptor cells are often selective to particular classes of stimuli, some are more broadly tuned to stimuli, i.e., less specific in their responses (may be excited by both salt and sour for example).
- Primary taste axons are even less specific than receptor cells, and most central taste neurons continue to be broadly responsive all the way into the cortex. In other words, the response of a single taste cell is often ambiguous about the food being tasted; the labels on the taste lines are uncertain rather than distinct.

Question 27

why are cells in the taste system broadly tuned?

Answer 27

-If one taste receptor cell has two different transduction mechanisms, it will respond to two types of tastants (although it may still respond most strongly to one of them).
-There is convergence of receptor cell input onto afferent axons. Each receptor cell synapses onto a primary taste axon that also receives input from several other receptor cells in that papilla as well as its neighbours. This means one axon may combine the taste information from numerous taste cells

Question 28

what is population coding?

Answer 28

-Responses of a large number of broadly tuned neurons, rather than a small number of precisely tuned neurons, are used to specify the properties of a particular stimulus such as taste
-in the case of taste, receptor cells are sensitive to a small number of taste types, often only one; gustatory axons and the neurons they activate in the brain tend to respond more broadly— for example, strongly to bitter, moderately to sour and salt, and not at all to sweet.
-Only with a large population of taste cells, with different response patterns, can the brain distinguish between specific alternative tastes.

Question 29

what do we smell with?

Answer 29

small, thin sheet of cells high up in the nasal cavity called the olfactory epithelium

Question 30

describe the olfactory epithelium

Answer 30

-has 3 main cell types
–>olfactory receptors cells, sites of transduction
–>supporting cells, similar to glia, among other things, help produce mucus
–>basal cells, source of new receptor cells

-Olfactory receptors (similar to taste receptors) continually grow, die, and regenerate, in a cycle that lasts about 4– 8 weeks.

Question 31

what are the chemical stimuli in the air called

Answer 31

odorants

Question 32

what do odorants do as they pass the olfactory epithelium before reaching receptor cells?

Answer 32

dissolves in the mucus layer before they reach receptor cells

Question 33

describe olfactory receptor neurons

Answer 33

-have single thin dendrite that ends with a small knob at the surface of the epithelium
-Waving from the knob, within the mucus layer, are several long, thin cilia. Odorants dissolved in the mucus bind to the surface of the cilia and activate the transduction process. On the opposite side of the olfactory receptor cell is a very thin, unmyelinated axon. Collectively, the olfactory axons constitute the olfactory nerve (cranial nerve I).

Question 34

What happens to the olfactory axons after leaving the epithelium?

Answer 34

small clusters of axons penetrate thin sheet of bone called cribriform plate, then course into the olfactory bulb

Question 35

Describe olfactory transduction pathway

Answer 35

Odorants → Bind to membrane odorant receptor proteins → Stimulate G-protein (G olf ) → Activate adenylyl cyclase → Form cAMP → Bind cAMP to a cyclic nucleotide-gated cation channel → Open cation channels and allow influx of Na + and Ca 2+ → Open Ca 2+ -activated Cl – channels → Cause current flow and membrane depolarization (receptor potential).

Question 36

What happens once cation-selective cAMP-gated channels open in olfactory receptors?

Answer 36

-Current flows inward, and the membrane of the olfactory neuron depolarizes.
-In addition to Na + , the cAMP-gated channel allows substantial amounts of Ca 2+ to enter the cilia. In turn, the intracellular Ca 2+ triggers a Ca2+-activated Cl – current that may amplify the olfactory receptor potential. (a switch from usual effect of Cl- which is to inhibit, internal Cl- concentration must be unusually high)

Question 37

describe olfactory receptor proteins

Answer 37

-have odorant binding site on their extracellular surface
-have more than 350 different odorant receptors gene in humans
-G-proteins couple receptors –> all have 7 transmembrane alpha helices –> only second messenger mediating olfactory transduction in vertebrates is cAMP

Question 38

how does olfactory discrimination occur?

Answer 38

-involves population-coding scheme
(1) Each odour is represented by the activity of a large population of neurons
(2) the neurons responsive to particular odours may be organized into spatial maps
(3) the timing of action potentials may be an essential code for particular odours.

Question 39

where does olfactory neurons send axons to?

Answer 39

into the 2 olfactory bulbs

Question 40

describe the input of each bulb

Answer 40

-contains spherical structures called glomeruli (50-200 micrometres in diameter)
- Within each glomerulus, the endings of about 25,000 primary olfactory axons (axons from the receptor cells) converge and terminate on the dendrites of about 100 second-order olfactory neurons
-each glomerulus receives input from only receptors cells of one particular type –> i.e., a map of odour information

Question 41

what is olfactory information modified by?

Answer 41

-Modified by inhibitory and excitatory interactions within and among the glomeruli and between the two bulbs.
-Neurons in the bulbs are also subject to modulation from systems of axons that descend from higher areas of the brain.

Question 42

what most important brain regions receives input from olfactory bulb? describe this

Answer 42

-The olfactory cortex and some of its neighbouring structures in temporal lobe
-does not go through thalamus like the other senses
-produces unusually direct and widespread influence on parts of forebrains that have roles in odour discrimination, emotion, motivation and memory

Question 43

describe olfactory population coding

Answer 43

• use of responses of a large population of receptors to encode a specific stimuli –> looing at combination of responses to find specific stimuli

Question 44

describe the olfactory maps

Answer 44

-a sensory map is an orderly arrangement of neurons that correlates with certain features of the environment
-Microelectrode recordings show that many receptor neurons respond to the presentation of a single odorant and that these cells are distributed across a broad area of the olfactory epithelium.
-However, we have seen that the axons of each receptor cell type synapse upon particular glomeruli in the olfactory bulbs. Such an arrangement yields a sensory map in which neurons in a specific place in the bulb respond to particular odours.
-Thus, the smell of a particular chemical is converted into a specific map defined by the positions of active neurons within the “neural space” of the bulbs.

Question 45

Describe temporal coding in the olfactory system and example of this

Answer 45

-depends on the timing of spikes, might instead encode the quality of odours
-loss of synchronous spiking was associated with a loss of bees’ ability to discriminate between similar odours, although not between broad categories of odours. The implication is that the bee analyses an odour not only by keeping track of which olfactory neurons fire but also by when they fire.